Optical sheet controlling external light, display device and method for producing optical sheet

ABSTRACT

An optical sheet, which can suitably absorb external light over a wide range and can improve a contrast, a display device, and a method for producing an optical sheet. The optical sheet is disposed on an observer side relative to an image light source and includes a plurality of layers that control light emitted from the image light source to emit the light on the observer side, at least one of the plurality of layers is an optical functional sheet layer which includes prisms being arranged in parallel along the surface of the optical sheet whereby light can be transmitted and wedge portions are being arranged in parallel between the prisms whereby light can be absorbed. At least one of the plurality of layers other than the optical functional sheet layer is a light-absorbing layer.

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/677,331, filed Mar. 10, 2010, which is a national stageentry of PCT/JP2008/066311, filed Sep. 10, 2008, which claims foreignpriority from Japanese Patent Application No. 2007-237142, filed on Sep.12, 2007, which is incorporated by reference in its; entirety.

TECHNICAL FIELD

This invention relates to an optical sheet, which is disposed on anobserver side relative to an image light source and can suitably controlimage light and external light, a display device, and a method forproducing an optical sheet. More particularly, this invention relates toan optical sheet, which cars block external light entering over a widerange to improve the contrast, a display device, and a method forproducing an optical sheet.

BACKGROUND ART

In a display device such as a television using a plasma display panel(hereinafter, it may be referred to as “PDP”), an optical sheet isdisposed on an observer side relative to the PDP. The optical sheetserves to provide the observer with a higher quality image. Therefore,the optical sheet includes prisms which are arranged in parallel alongthe sheet surface of the optical sheet and transmit therethrough imagelight from the PDP. Further, the optical sheet includes wedge portionsprovided between the prisms and suitably blocking or reflecting imagelight and external light to improve a contrast or to control ghost (seePatent Document 1).

Patent Document 1; Japanese Patent Application Laid-Open No. 2006-189367

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the optical sheet disclosed in Patent Document 1, external light isabsorbed only by the wedge portions disposed between the prisms. Thus,the optical sheet can absorb well external light (for example, lightfrom ceiling lighting) entering at a large angle with respect to thenormal line of a screen. However, the optical sheet is less likely toabsorb external light entering at a small angle with respect to thenormal line of the screen. Therefore, the contrast, sometimes cannot besatisfactorily obtained as a whole.

Thus, in view of the above problem, an object of the present inventionis to provide an optical sheet, which can suitably absorb external lightover a wide range and can improve the contrast, the display device, anda method for producing an optical sheet.

Means for Solving the Problem

Hereinafter, the present invention will be described. For ease ofunderstanding the invention, the reference numerals of the attacheddrawings are enclosed in parenthesis, but by no means limit theinvention to the illustrated embodiments.

The first aspect of the present invention provides an optical sheet (10,20, 30) disposed on an observer side relative to an image light sourceincluding: a plurality of layers that control light emitted from theimage light source to emit the light on the observer side, wherein atleast one of the plurality of layers is an optical functional sheetlayer (11, 21, 31), which includes prisms (12, 22, 32) being arranged inparallel along a surface of the optical sheet whereby light can betransmitted and wedge portions (13, 23, 33) being arranged in parallelbetween the prisms whereby light can be absorbed; and at least one ofthe plurality of layers other than the optical functional sheet layer isa light-absorbing layer (17, 27, 37) capable of absorbing light, therebythe above problem is solved.

The second, aspect of the invention is characterized in that thelight-absorbing layer (17, 27, 37) of the optical sheet (10, 20, 30)according to the first aspect of the invention absorbs light having apredetermined wavelength.

The third aspect or the invention is characterized in that the wedgeportion (13, 23, 33) and the light-absorbing layer (17, 27, 37) of theoptical sheet (10, 20, 30) according to the first or second aspect ofthe invention comprise the same material.

The fourth aspect of the invention is characterized in that thelight-absorbing layer (17, 27, 37) of the optical sheet (10, 20, 30)according to any one of the first, to third aspects of the invention isformed integrally with the wedge portion (13, 23, 33).

The fifth aspect of the invention is characterized in that the wedgeportion (13, 23, 33) of the optical sheet (10, 20, 30) according to anyone of the first to fourth aspects of the invention is configured toshow a light absorption performance having a transmittance of 40-70%when measuring the transmittance of a 6 μm thick optical sheet made onlyof materials for forming the wedge portion.

The “transmittance” means the ratio of the brightness before and afterthe target sheet is disposed and it has a value of up to 100%.

The sixth aspect of the invention is characterised in that in a crosssection in a sheet thickness direction of the optical sheet (10, 20, 30)according to any one of the first to fifth aspect of the invention, theprism (12, 22, 23) of the optical, functional sheet layer (11, 21, 31)has a trapezoidal shape whose short upper base is located at one sheetsurface side and long lower base is located at the other sheet surfaceside, and the wedge portion (13, 23, 33) has a triangular shape whosebottom, is located at the one sheet surface side.

The seventh aspect of the invention is characterized in that in. a crosssection in a sheet thickness direction of the optical sheet (10, 20, 30)according to any one of the first to fifth aspects of the invention, theprism (12, 22, 32) of the optical functional sheet layer (11, 21, 31)has a trapezoidal shape whose short upper base is located at one sheetsurface side and long lower base is located at the other sheet surfaceside, and the wedge portion (13, 23, 33) has a trapezoidal shape whoselong lower base is located at the one sheet surface side and short upperbase is located at the other sheet surface side.

The eighth aspect of the invention is characterized in that the obliqueline of the trapezoid provided between the upper base and the lower baseof the prism (12, 22, 32) of the optical sheet. (10, 20, 30) accordingto the sixth or seventh aspects of the invention has an angle of morethan 0° and not more than 10° with respect to the normal line of thesheet surface.

The ninth aspect, of the invention is characterised in. that the obliqueline of the optical sheet (10, 20, 30) according to eighth aspect of theinvention, includes a polygonal line or a curved line.

When the oblique line is curved, the angle formed by the curved line andthe normal line of the sheet surface is obtained as follows. The curvedline is divided equally into 10 lines in the sheet thickness direction,and the end points of the obtained curved lines are connected to obtainstraight lines. An angle formed by each of the straight lines and thenormal line of the sheet surface is more than 0° and not more than 10°.

The 10th aspect of the invention is characterized in that the wedgeportion (13, 23) of the optical sheet (10, 20) according to any one ofthe first to ninth aspects of the invention contains light-absorbingparticles with an average particle diameter of not less than 1 μm.

With regard to “the average particle diameter is 1 μm” in the phrase “anaverage particle diameter of not less than 1 μm”, a target, particle hasa diameter of not less than 0.5 μm and less than 1.5 μm when, measuringthe particle size using a weight, distribution method, and a standard,deviation is not. less than 0.3 in particle size distribution.

The 11th aspect of the invention is characterized in that a materialconstituting the prism (12, 22, 32) of the optical sheet (10, 20, 30)according to the first to 10th aspects of the invention has a refractiveindex of Np, a material constituting the wedge portion (13, 23, 33) hasa refractive index of Nb, the Np and Nb have a relation of Np ≧Nb, andthe Np and Nb have a value in a range of 1.49 to 1.56.

The 12th aspect of the invention is characterized in that the prism andthe wedge portion of the optical functional sheet layer (11, 11″) of theoptical sheet (10″) according to any one of the first to 11th aspects ofthe invention each have a predetermined cross section and extend in thelongitudinal direction, and the two optical functional sheet layers arestacked such that the wedge portions of the two optical functional sheetlayers are perpendicular to one another in the longitudinal directions.

The 13th aspect of the invention provides a display device (1)characterized by including the optical sheet (10, 20, 30) according toany one of the first to 12th aspects of the invention, whereby the aboveproblem can be solved.

The 14th aspect of the invention provides a method for producing theoptical sheet (10, 20, 30) according to any one of the first to 12thaspects of the invention. The process includes a step of forming thelight-absorbing layer (17, 27, 37) by coating. The step includes a stepof applying a light-absorbable resin to be cured by application of lightand a step of irradiating light to the applied light-absorbable resin tocure the light-absorbable resin. The production process can solve theabove problem.

Effect of the Invention

The present invention can provide: an optical sheet which can suitablyabsorb external light over a wide range and can improve a contrast; anda display device including the optical sheet.

The present invention also provides a production method, which includesapplying a light-absorbable material to cure the material for producingan optical sheet, which makes it possible to easily form thelight-absorbing layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical sheet according to afirst embodiment of the present invention and shows a schematic layerstructure of the optical sheet;

FIG. 2 is a partially enlarged view of the optical sheet of FIG. 1;

FIG. 3 is a partially enlarged view of an optical sheet according to avariation of the invention;

FIG. 4 is a partially enlarged view of the optical sheet of theinvention when a wedge portion has a trapezoidal shape;

FIG. 5 is a view showing a layer structure of the optical sheet of theinvention when two optical functional sheet layers are stacked;

FIG. 6 is a cross-sectional view of an optical sheet according to asecond embodiment of the invention and shows a schematic layer structureof the optical sheet;

FIG. 7 is a cross-sectional view of an optical sheet according to athird embodiment of the invention and shows a schematic layer structureof the optical sheet;

FIG. 8 is a partially enlarged view of the optical sheet of FIG. 7;

FIG. 9 is a cross-sectional view of an optical sheet which is avariation of the optical sheet of FIG. 7 and shows a schematic layerstructure of the optical sheet;

FIG. 10 is a view focusing on a part of a plasma television where POPand the optical sheet of the present invention are provided, when theoptical sheet is attached to the plasma television;

FIG. 11 is a view showing a relation between a view angle and a relativeluminance;

FIG. 12 is a view for explaining external light absorption in theoptical sheet of FIG. 1;

FIG. 13 is a view for explaining external, light absorption in theoptical sheet of FIG. 6; and

FIG. 14 is a view for explaining external light absorption, in aconventional example.

EXPLANATION OF REFERENCE NUMERALS

1 Plasma television (display device)

2 Plasma display panel (PDP)

10, 20, 30 Optical sheet

11, 21, 31 Optical functional sheet layer

12, 22, 32 Prism

13, 23, 33 Wedge portion

14, 24, 34 Binder (binder portion)

15, 25, 35 Light absorbing particle

16, 26, 36 PET film layer

17, 21, 37 Light absorbing layer

18, 28, 38 Adhesive layer

Best. Modes for Carrying Out the Invention

The aforementioned functions and benefits of the present, invention willbe apparent from the following best modes for carrying out theinvention,

Hereinafter, the present invention will be described based on.embodiments shown in the drawings.

FIG. 1 shows a cross section of an optical sheet 10 of a firstembodiment according to the present invention and shows a schematiclayer structure of the optical sheet 10. In FIG. 1, some repeatedreference numerals may be omitted for viewability (the same holds forthe following drawings). The optical sheet 10 comprises; an opticalfunctional sheet layer 11, a PET film layer 16 as a base material layer,a light-absorbing layer 17, and an adhesive layer 18. These layersextend on the far/near side oil the drawing while maintaining the crosssection shown in FIG. 1. Each layer will be described as below.

The optical functional sheet layer 11 comprises prisms 12 and wedgeportions 13 arranged between the prisms 12. The cross section of theprism 12 perpendicular to the sheet surface of the optical sheet 10 hasa substantially trapezoidal shape. FIG. 2 is a cross-sectional enlargedview focusing on the wedge portion 13 and the prisms 12 adjacent to thewedge portion 13. The optical functional sheet layer 11 will bedescribed with reference to FIGS. 1 and 2 as well as other: suitabledrawings.

The prisms 12 are elements each of which has a substantially trapezoidalcross section whose upper base is located at one side of the opticalfunctional sheet layer 11, and lower base is located at the other side.The prisms 12 are respectively formed of a light transmissive resin witha refractive index of Np. This is usually formed of, for example, epoxyacrylate characterized by being cured by, e.g., ionizing radiation,ultraviolet. The magnitude of Np is not specifically limited; however,in terms of availability of material, the magnitude is preferably 1.49to 1.56. When image light transmits through the prisms 12, the imagelight is provided to an observer.

The wedge portions 13 are disposed between the prisms 12, Thus, each ofthe wedge portions 13 has a substantially triangular shape whose basecorresponds to the upper base of the prism 12, and apex facing the basecorresponds to the lower base of the prism 12. Each of the wedgeportions 13 comprises a binder-portion 14 filled with a material with arefractive index of Nb and light absorbing particles 15 mixed in thebinder 14. External light enters or is absorbed by the wedge portions13, whereby the contrast of an image can be improved.

A binder material filled in. the binder portion 14 is formed of amaterial with a refractive index of Nb not more than the refractiveindex of Kp of the prisms 12. The magnitude of Nb is not specificallylimited; however, in terms of availability of the material, themagnitude is preferably 1.49 to 1.56. Although the difference betweenthe refractive indexes Np and Nb is not particularly limited, thedifference is preferably 0 to 0.06. Although a material used as thebinder material is not particularly limited, for example, urethaneacrylate characterized by being cured by, e.g., ionizing radiation andultraviolet cam be used.

According to the relation between the refractive index difference andthe incident angle of image light, a part of the image light can bereflected on the interface without entering into the wedge portions(light absorbing parts) 13. Since the reflected light is provided to anobserver, a bright image can be provided.

The average diameter of the light absorbing particles 15 is preferablynot less than 1 μm in view of availability and handling. The lightabsorbing particles 15 are colored to a predetermined density by apigment such as carbon or a dye in red, blue, or yellow. The lightabsorbing particles 15 may be colored by commercial colored resin fineparticles. The refractive index Nr of the light absorbing particle 15 isnot specifically limited.

Although the light absorption performance of the wedge portions 13 canbe suitably adjusted according to the purposes, the wedge portions arepreferably configured to show a light absorption performance having atransmittance of 40-70% when measuring the transmittance of a 6 μm thickoptical sheet made only of materials for forming, the wedge portion.Although the means for attaining the transmittance of 40 to 70% is notparticularly limited, the content of light absorbing particles and thelight absorption performance may be adjusted.

An angle θ of the oblique sides (two sides extending in the sheetthickness direction) of the wedge portions 13 with respect to the normalline of the sheet surface is not specifically limited. However, in termsof suitable reflection/absorption of both external light and imagelight, in many cases, the angle is preferably more than 0° and not morethan 10°, and particularly more than 0° and not more than 6°.

As the shape of the optical functional sheet layer 11, as shown in FIGS.1 and 2, each of the prisms 12 has a substantially trapezoidal crosssection, and each of the wedge portions 13 held between the prisms 12has a triangular cross section. However, if light can be suitablycontrolled, those shapes are not specifically limited, and otherappropriate shapes are suitably used. FIG. 3 shows a variation. FIG. 3corresponds to FIG. 2 and is a view focusing on a wedge portion 13′ andprisms 12′ provided on the both sides of the wedge portion 13′. As seenin FIG. 3, the oblique lines in the cross section of the wedge portion13′ (the oblique lines of the prisms 12′) are constituted of not oneoblique line but two oblique lines 13 a′ and 13 b′. Namely, the wedgeportion. 13′ has polygonal line shaped oblique lines in the crosssection. Specifically, with regard to the oblique lines 13 a′ providedon the upper base side of the prism 12′ (the shorter base side) (theleft side of FIG. 3), the oblique line 13 a′ has an angle Ch withrespect to the normal line of the sheet surface of the optical sheet.Meanwhile, with regard to the oblique lines 13 b′ provided on the lowerbase side of the prism 12′ (the longer base side) (the right side ofFIG. 3), the oblique line 13 b′ has an angle θ₂ with respect to thenormal line of the sheet surface of the optical sheet.

The angles θ₁ and θ₂ have a relation of θ₁ >θ₂, and they preferably fallwithin a range of more than 0° and not more than 10°, and particularlymore than 0° and not more than 6°. The two oblique lines 13 a′ and 13 b′intersect with each other at a position dividing the thickness of theoptical functional sheet layer 11 into T₁ and T₂ (the left and rightdirections of FIG. 3). T₁ and T₂ are preferably the same thickness.

In this variation, the wedge portion 13′ has two oblique lines. However,the wedge portion 13′ is not limited thereto but may have three or moreoblique lines to thereby have a polygonal line shaped cross section ormay have curved oblique lines.

In the present embodiment, there has been described that the wedgeportions each have a triangular shape; however, the shape is not limitedthereto but may have a trapezoidal shape. FIG. 4 shows a trapezoidalwedge portion 13″ of an optical functional sheet layer 11″ and prisms;12″ adjacent thereto. In this example, the wedge portion 13″ has atrapezoidal shape shown in FIG. 4. In that case, the longer base (thelower base) of the trapezoid can be provided on the side remote from aPET film layer (not shown) (the left side of FIG. 4), and the shorterbase can be provided on the PET film layer side (the right side of FIG.4). The length B of the upper base of FIG. 4 preferably falls in a rangeof 2 to 25 μm.

Returning to FIG. 1, other constitution of the optical sheet 10 will bedescribed. The PET film layer 16 is a film layer as a base for formingthe optical functional sheet layer 11 on the PET film layer 16. The PETfilm layer 16 is composed mainly of PET. As long as the PET film layer16 is composed mainly of PET, it may contain other resins. The maincomponent of the PET film layer 16 is a component contained in an amountof not less than 50 wt. % based on the weight of the entire PET filmlayer 16. The PET film layer 16 may further contain a suitable amount ofvarious additives. Conventional additives include an antioxidant ofphenol type or the like and a stabilizer of lactone type or the like.

The light-absorbing layer 17 is a layer reducing the amount of light.The light-absorbing layer is not specifically limited as long as it hasa property capable of reducing the amount of light. In the reduction ofthe amount of light, the amount of light may be reduced regardless of awavelength of the light, or the amount of light with a wavelength withina predetermined range may be reduced. When the amount of light with apredetermined wavelength is reduced, the so-called color tone can becorrected; thereby the reduction in the light amount and the color tonecorrection can be simultaneously attained. Hence convenience can beenhanced.

When the amount of light is reduced regardless of a wavelength of light,for instance, ND filter (Neutral Density Filter) layer can be used. Whenthe amount of light with a wavelength within, a predetermined range isreduced, a Tint layer or the like can be used.

The amount of light reduced in the light-absorbing layer 17 can besuitably selected by the brightness of applicable PDP and a displaydevice. As an example, the thickness and light-absorbing performance ofthe light-absorbing layer 17 can be adjusted to make the transmittanceof the light-absorbing layer 17 be approximately 60%.

The adhesive layer 18, as described later, is, for example, a layerdisposed with an adhesive for bonding the optical sheet 10 to othersheets or members disposed in a plasma television 1. An adhesive used inthe adhesive layer 18 transmits light therethrough; thus, as long as theadhesive can suitably bond the optical sheet 10 to other components, thematerial is not particularly limited. For example, acrylic-typecopolymer can be used, and the viscosity is, for example, approximatelyseveral. N/25 mm to 20N/25 mm.

The optical sheet 10 having the above constitution can. block widespreadexternal light, thereby the optical, sheet attaining the improvement ofa contrast can be provided. The reason will be described later.

FIG. 5 is a cross-sectional view of an optical sheet 10′″ of anothervariation of the present invention and shows a schematic layer structureof the optical sheet 10′″. The optical sheet 10′″ of this variationincludes the other optical functional sheet. layer 11′″ held between theoptical functional sheet layer 11 and the PET film layer 16 of theoptical sheet 10 of the first embodiment. In this case, a prism 12′″ anda wedge portion (not shown) are disposed perpendicular to the prisms 12and the wedge portions 13 of the optical functional sheet layer 11.Thus, the prism 12′″ of the optical functional sheet layer 11′″ and thewedge portion are alternately arranged in parallel on the far/near sideof FIG. 5.

FIG. 6 is a cross-sectional view of an optical sheet 20 of the presentinvention according to a second embodiment and shows a schematic layerstructure of the optical sheet 20. The optical sheet 20 comprises: anoptical functional sheet layer 21, a PET film layer 26 as a basematerial layer, a light-absorbing layer 27, and an adhesive layer 28,These layers extend on the far/near side of the drawing whilemaintaining the cross section shown in FIG. 6.

The optical sheet 20 of the second embodiment is characterized in thatthe light-absorbing layer 27 is provided on one side of the opticalfunctional sheet layer 21, which is the opposite side of the opticalfunctional sheet layer 21 from the PET film layer 26. Even when thelight-absorbing layer 27 is disposed in this manner, the optical sheet20 is applicable to the optical sheet of the present invention. Sincethe layers of the optical sheet 20 are in common with those in theoptical sheet 10, their descriptions will be omitted here.

The optical sheets 10 and 20 are, for example, produced as follows. Aliquid body which is a material of prisms is applied to each one side ofthe PET film layers 16 and 26. Then, while holding the material of theprisms between a PET film and a roll die for forming the shape of theprism, ultraviolet is irradiated to cure the material to form the prisms12 and 22. Then, to the gap between the prisms 12, 12 and 22, 22, aliquid body having a transparent resin as a material of a bindercontaining black light absorbing particles is filled. Extra material isthen removed by, for example, squeezing. Ultraviolet is then irradiatedto cure the liquid body, to form the wedge portions 13 and 23.Consequently, the optical functional sheet layers 11 and 21 areproduced. The light-absorbing layers 17 and 27 as well as the adhesivelayers 18 and 28 are respectively stacked on the optical functionalsheet layers 11 and 21.

The method for stacking the light-absorbing layers 17 and 27 is notparticularly limited. For example, a layer which has been, previouslyformed into a film form and becomes a. light-absorbing layer is stacked,whereby the light-absorbing layer may be formed. Alternatively, a lightcuring material to become a light-absorbing layer is coated onto a sheetin the form of a liquid body, ultraviolet is irradiated to the materialto cure the material, whereby the light-absorbing layer may be formed.According to this constitution, the light-absorbing layer can bedirectly coated.

FIG. 7 is a cross-sectional, view of an optical, sheet 30 according tothe third embodiment of the present invention and shows a schematiclayer structure of the optical sheet 30. FIG. 8 is a partially enlargedview of FIG. 7. The optical sheet 30 comprises: an optical functionalsheet layer 31, a PET film layer 36 as a base material layer, alight-absorbing layer 37, and an adhesive layer 38, These layers extendon the far/near side of the drawing while maintaining the cross sectionshown in FIG. 7.

As seen in FIGS. 7 and 8, the optical sheet 30 of the third embodimentfurther comprises the light-absorbing layer 37 on one side of theoptical functional sheet layer 31, which is the opposite side of theoptical functional sheet layer 31 from the PET film layer 36. Binders 34of wedge portions 33 are formed of the same material as the material ofthe light-absorbing layer 37 to have a light-absorbing action. Thelight-absorbing layer 37 and the wedge portions 33 are integrally formedwith each other. Even when the light-absorbing layer 37 is disposed inthis manner, the optical sheet 30 is applicable to the optical sheet ofthe present invention. Since the layers of the optical sheet 30 are incommon with those in the optical sheet 10, their descriptions will beomitted here. According to this constitution, the binders 34 can have aneffect of reducing the amount of light and/or an effect of color tonecorrection.

The thickness of the light-absorbing layer 37 is not specificallylimited as long as necessary performance can be obtained. However, inview of manufacturing, it is preferable that the light-absorbing layer37 has a thickness not more than the diameter of the light-absorbingparticles contained in the wedge portions 33. A material of a binder ofthe light-absorbing layer 37 and the wedge portions 33 is notspecifically limited; for example, urethane acrylate containing blackpigment can be used. Examples of the black pigment include carbon black,ivory black, aniline black, vine black, peach black, and lamp black.

The optical sheet 30 is, for example, produced as follows. A liquid bodywhich is a material of the prisms is applied, to one side of the PETfilm layer 36. Then, while holding the material of the prisms between, aPET film and a roll die for forming the shape of the prism, ultravioletis irradiated to cure the material, to form the prisms 32. Then, abinder material dispersed with black light-absorbing particles andhaving a light-absorbing action is filled, in between the prisms 32; andthereafter, the binder material is coated onto the surface of the upperbase side of each of the prisms 32. Extra material is then removed fromthe surface on the upper base side of each of the prisms 32 by squeezingsuch that the dispersed light-absorbing particles do not remain and thebinder material forms a layer with a predetermined thickness byadjusting a gap. Then, ultraviolet is irradiated to cure the resin toform the wedge portions 33 and the light-absorbing layer 37. Accordingto this constitution, the wedge portions and the light-absorbing layercan be simultaneously formed.

In this embodiment, the wedge portions 33 and the light-absorbing layer37 are simultaneously cured; however, the same materials are notnecessarily cured simultaneously, and the wedge portions and thelight-absorbing layer may be cured separately.

FIG. 9 is a cross-sectional view of an optical sheet 30′ which is avariation of the optical sheet 30 and shows a schematic layer structureof the optical sheet 30′. The optical sheet 30′ comprises: an opticalfunctional sheet layer 31′, a PET film layer 36′ as a base materiallayer, a light-absorbing layer 37′, and an adhesive layer 38′. Theselayers extend on the far/near side of the drawing while maintaining thecross section shown in FIG. 9.

The optical sheet 30′ does not include the light-absorbing particles 35of the optical sheet 30 (see FIG. 8.) but does include a binder resinhaving a light-absorbing action. The optical sheet 30′ can also beapplied to the optical sheet of the present invention. Since the layersof the optical sheet 30′ other than the light-absorbing particles are incommon with those in the optical sheet 30, their descriptions will beomitted here.

Each base material layer of the optical sheets 10, 20, 30, and 30′ ofthe present invention is not necessarily formed of PET, and “apolyester-based resin” such as polybutylene terephthalate resin (PBT)and polytrimethylene terephthalate resin (PTT) may be used. In thepresent embodiments, in view of performance, mass productivity, price,availability, and so on, a resin composed mainly containing PET ispreferably used as the material of the base material layer.

The optical sheet of the present, invention may further comprisefunctional film layers such as a film layer blocking electromagneticwaves, a film layer correcting a color tone, a film layer cutting a neonline, a film layer cutting infrared rays, a film layer preventingreflection, a film layer preventing static charge, and a film layer foranti glare.

Next, the constitution of the optical sheet 10 of the present inventionattached to the plasma television 1 which is a display device will bedescribed. FIG. 10 is a cross-sectional view focusing on a part of theplasma television 1 where the PDP 2 and the optical sheet 10 aredisposed when disposing the optical sheet 10 on the light emission sideof the PDP 2 and assembling the PDP 2 and the optical sheet. 10 in theplasma television 1. The right side of FIG. 10 is an observer side. Inthis example, the optical sheet 10 is used, but the same holds for theoptical sheets 20 and 30.

As shown in FIG. 10, the optical sheet 10 of the present invention isdisposed on the observer side at an interval “A” with respect to the PDP2 as an image source. The observer side of the adhesive layer 18 has aglass plate 6 and various layers (3, 4, and 5) having functions of AR,AS, and AG. “AR” stands for “antireflection” and is a function ofsuppressing a light reflectance. “AS” stands for “antistatic” and is astatic charge prevention function. “AG” stands for “anti-glare” and is afunction capable of preventing glare of the surface of a prism.

In the above example, the optical sheet further comprises various layerof AR, AS, and AG. However, as required, the optical sheet canfurthermore comprise layers having other functions: such as a layerblocking electromagnetic waves, a layer blocking infrared rays and aneon line, and a toning layer.

Although the interval “A” is provided, between the PDP 2 and the opticalsheet 10; it is not necessarily provided, and the optical sheet 10 maybe stacked directly on the surface of the PDP 2. In this case, thesurface of the optical sheet 10 facing the PDP 2 includes an adhesivelayer.

Next, how external light is blocked by the optical sheet of the presentinvention will be described. FIG. 11 is a graph of the so-called “viewangle property” showing each property of a light-absorbing layer and anoptical functional sheet layer when the horizontal axis represents aview angle (degree) and the vertical axis represents a relativeluminance (%). The view angle means an angle formed by the normal lineof a screen from the center of the screen and a line of sight toward thecenter of the screen. With regard to the view angle, the upper side ofFIG. 10 is positive, and the lower side of FIG. 10 is negative. Therelative luminance means the ratio (percentage) of the luminance in thecase where the optical sheet is disposed to the luminance of 100% in thecase where the optical sheet is not disposed.

The view angle properties are obtained by measuring the luminance ateach view angle. In the measurement, a three-dimensional goniophotometerGP-500 (manufactured by Murakami Color Research Laboratory Co., Ltd.) isused.

As seen in FIG. 11, when the view angle is ±15 degrees, the opticalfunctional sheet layer has a relative luminance higher than the relativeluminance of the light-absorbing layer. Thus, this means the opticalfunctional sheet layer is less likely to absorb external light in arange of the relevant view angle. Meanwhile, the relative luminance ofthe light-absorbing layer is approximately constant in a wide range.However, therefore the relative luminance is high at a large view angle(the angle with a large absolute value), and external light is poorlyabsorbed.

According to the optical sheet 10 of the present invention, asschematically shown in FIG. 12, external light L11 entering at a largeangle (the angle with a large absolute value) is absorbed mainly by thewedge portion 13. Meanwhile, external light L12 entering at a smallangle (the angle with a small absolute value) is absorbed mainly by thelight-absorbing layer 17. FIG. 13 shows a similar example using theoptical sheet 20. In FIG. 13, external light L21 entering at a largeangle (the angle with a large absolute value) is absorbed mainly by thewedge portion 23. Meanwhile, external light L22 entering at a smallangle (the angle with, a small absolute value) transmits through theprism 22 but is absorbed by the light-absorbing layer 27.

Meanwhile, according to the conventional optical sheet 110, as shown inFIG. 14, external light L111 entering at a large angle (the angle with alarge absolute value) is absorbed mainly by a light absorbing particle115 in a wedge portion 113 of an optical functional sheet layer 111;however, external light L112 entering at a small angle (the angle with asmall absolute value) is not absorbed but transmits through a prism 112of the optical sheet 110. Thus, the transmitted, light reaches the PDFor is reflected by other surfaces or boundaries, whereby the lightinfluences image light to lead to reduction in contrast.

As described above, according to the optical sheet of the presentinvention, external light entering from a wide range can be suitablyabsorbed. Thus, a good contrast optical sheet can be provided.

Examples of the optical sheet having the above constitution will bedescribed in more detail as follows. However, the present invention isnot limited to the range of the examples.

EXAMPLES

In this example, with regard to an optical sheet including awavelength-dependent light-absorbing layer (example) and an opticalsheet without including a light-absorbing layer (comparative example),the influences on a contrast given when decreasing the external lightincident angle are tested. Hereinafter, the conditions and results areshown.

<Test Samples>

As a test example, an optical sheet in which only the presence of alight-absorbing layer and the stacked position are changed is produced.The specifics are shown in Table 1.

TABLE 1 Occupied Slope Thickness Pitch of area ratio angle of of opticalwedge of wedge wedge functional Light- portions portion portion sheetlayer absorbing No. (mm) (%) (degree) (mm) layer Notes 1 70 30 4.5 100Presence Example (observer side) 2 70 30 4.5 100 Presence Example (imagesource side) 3 70 30 4.5 100 Absence Compar- ative example

The optical sheet No. 1 includes a light-absorbing layer on the observerside (corresponding to the optical sheet 10), and the optical sheet No.2 includes a light-absorbing layer on the image source side(corresponding to the optical sheet 20). The optical sheet No. 3 doesnot include a light-absorbing layer.

<Contrast Measurement>

The contrast was measured as follows. External light was irradiated fromabove to the center of a screen of a PDP at a predetermined angle suchthat the illuminance level was 150 lx. The contrast was the ratio of thebrightness when the screen was displayed in white by the PDP to thebrightness when it was displayed in black. The predetermined angle was30°.

In the evaluation, a value of a. contrast at the predetermined angle(30°) and a value of a contrast at the external light incident angle of45° (a reference contrast) in the case where a light-absorbing layer isnot provided in an optical sheet (the same as the optical sheet No. 3)are compared with each other. The result, is shown in Table 2.

TABLE 2 Contrast External light Percentage of incident angle referenceNo. 30° contrast (%) Notes 1 56.5 103.5 Example 2 55.6 101.8 Example 342.3 77.5 Comparative example

The contrast at the external light incident angle of 45°, as a referencecontrast, in the case where the light-absorbing layer is not provided is54.6. Meanwhile, with regard to the contrast of each optical sheet atthe small external light incident, angle of 30°, the contrast of theoptical sheet No. 3 of the comparative example was lowered. Meanwhile,the optical sheets 1 and 2 of the present invention, regardless of smallexternal light, incident angle, could obtain the contrast substantiallythe same as the contrast at the external light incident angle of 45° ofthe optical sheet No. 3. Namely, the optical sheet including thelight-absorbing layer can obtain a good contrast even at a small angle,whereby a good contrast can be obtained even with respect to a widerange of external light incident angles.

The above has described the present invention associated with the mostpractical and preferred embodiments thereof. However, the invention isnot limited, to the embodiments disclosed in the specification. Thus,the invention can be appropriately varied as long as the variation isnot contrary to the subject substance and conception of the inventionwhich can be read out from the claims and the whole contents of thespecification. It should be understood that an optical sheet, a displaydevice, and a method for producing the optical sheet, with such analternation are included in the technical scope of the invention.

1. An optical sheet disposed on an observer side relative to an imagelight source comprising: a plurality of layers that control lightemitted from the image light source to emit the light on the observerside, wherein at least one of the plurality of layers is an opticalfunctional sheet layer, which comprises prisms being arranged inparallel along a surface of the optical sheet whereby light istransmitted and wedge portions being arranged in parallel between theprisms whereby light is absorbed; and at least one of the plurality oflayers other than the optical functional sheet layer is alight-absorbing layer capable of absorbing light all over thelight-absorbing layer, and is capable of absorbing the part of visiblelight; the light-absorbing layer and the wedge portions of the opticalfunctional sheet layer absorb a same wavelength of light; and thelight-absorbing layer is formed integrally with the wedge portion. 2.The optical sheet according to claim 1, wherein the light-absorbinglayer absorbs light having a predetermined wavelength.
 3. The opticalsheet according to claim 1, wherein the wedge portion is configured toshow a light absorption performance having a transmittance of 40-70%when measuring the transmittance of a 6 μm thick optical sheet made onlyof materials for forming the wedge portion.
 4. The optical sheetaccording to claim 1, wherein in a cross section in a sheet thicknessdirection, the prism of the optical functional sheet layer has atrapezoidal shape whose short upper base is located at one sheet surfaceside and long lower base is located at the other sheet surface side, andthe wedge portion has a triangular shape whose bottom is located at theone sheet surface side.
 5. The optical sheet according to claim 1,wherein in a cross section in a sheet thickness direction, the prism ofthe optical functional sheet layer has a trapezoidal shape whose shortupper base is located at one sheet surface side and long lower base islocated at the other sheet surface side, and the wedge portion has atrapezoidal shape whose long lower base is located at the one sheetsurface side and short upper base is located at the other sheet surfaceside.
 6. The optical sheet according to claim 4, wherein an oblique lineof the trapezoid provided between the upper base and the lower base ofthe prism has an angle of more than 0° and not more than 10° withrespect to a normal line of the sheet surface.
 7. The optical sheetaccording to claim 6, wherein the oblique line comprises a polygonalline or a curved line.
 8. The optical sheet according to claim 1,wherein the wedge portion contains light-absorbing particles with anaverage particle diameter of not less than 1 μm.
 9. The optical sheetaccording to claim 1, wherein a material constituting the prism has: arefractive index of Np; a material constituting the wedge portion has arefractive index of Nb; the Np and Nb have a relation of Np ≧Nb; and theNp and Nb have a value in a range of 1.49 to 1.56.
 10. The optical sheetaccording to claim 1, wherein the prism and the wedge portion of theoptical functional sheet layer each have a predetermined cross sectionand extend in a longitudinal direction and the two optical functionalsheet layers are stacked such that the wedge portions of the two opticalfunctional sheet layers are perpendicular to one another in longitudinaldirections.
 11. A display device comprising the optical sheet accordingto claim
 1. 12. A method for producing the optical sheet according toclaim 1, comprising: a step of forming the light-absorbing layer bycoating, wherein the step comprises a step of applying alight-absorbable resin to be cured by application of light and a step ofirradiating light to the applied light-absorbable resin to cure thelight-absorbable resin.
 13. The optical sheet according to claim 5,wherein an oblique line of the trapezoid provided between the upper baseand the lower base of the prism has an angle of more than 0° and notmore than 10° with respect to a normal line of the sheet surface. 14.The optical sheet according to claim 13, wherein the oblique linecomprises a polygonal line or a curved line.